1. Field of the Invention
The present invention relates to stable gel compositions containing a water-soluble salt of carboxymethyl cellulose (hereinafter abbreviated as CMC), particularly, uniform and stable gel compositions containing CMC which are useful for many purposes including preparation of medicines, cosmetics and the like, and a process for the preparation of such compositions.
2. Brief Description of the Prior Art
It is known that CMC is precipitated or gelated promptly by most polyvalent metal salts. In many instances, however, the product becomes fibrous or granular precipitates or solid massive precipitates and so the whole system becomes non uniform which is far from a uniform gel such as konnyaku jelly (devil's tongue jelly) or pudding.
For the formation of a uniform gel of synthetic macromolecules, there is a known process in which there is used a gelating agent that is hardly soluble in water, such as basic aluminum acetate, (Japanese Patent Laid-open No. Sho 54-106598). When this known process is applied to CMC, however, CMC is gelated on the surface of the gelating agent to form a big massive gel and the whole system results, in many cases, in a non-uniform one. Moreover, there are not many different such polyvalent metal salts that are hardly soluble in water. It is also possible to increase the vigor of the stirring on mixing the gelating agent. To increase the stirring, however, a special powerful stirring apparatus is required. Moreover, the product obtained by such powerful stirring is found, upon microscopic inspection to be nothing but a rather non-uniform one in which solid massive precipitates are divided into and dispersed in the form of fine granules.
We have, after studying processes for uniformly gelating CMC, found a process for preparing a very uniform and stable CMC gel, without using any special powerful stirring apparatus, by reacting CMC with a variety of water-soluble polyvalent metal salts, and have confirmed that the CMC gel so obtained is applicable for many practical uses including preparations of medicines, cosmetics and the like.
That is to say, we have succeeded in obtaining a very uniform and stable CMC gel free from solid mass or precipitates, by adding CMC wetted by or dispersed in a hydrophilic organic liquid compatible with water into an aqueous solution containing a water-soluble polyvalent metal salt.
It is considered that the gelation of CMC by a polyvalent metal salt is, in essence, a cross linking reaction by ionic bonds between the carboxyl groups of the CMC molecule and the polyvalent metal ions. In an aqueous solution prepared by dissolving CMC in water, almost all the molecules of CMC are uniformly dispersed and dissolved in water and so the carboxyl groups of CMC are in a highly reactive state by dissociation, for example, of sodium ions. Therefore, when a water-soluble polyvalent metal salt dissolved in water or in the form of powder is added to such an aqueous CMC solution, solid massive precipitates are formed in part and the gel obtained is not uniform, because the gelation velocity of CMC with the metal salt is much higher than the diffusion velocity of the metal salt. The gelation is not uniform when an aqueous solution of CMC is added to an aqueous soluton of a polyvalent metal salt, for the same reason.
On the other hand, neither precipitation nor gelation occurs at all when CMC and a water-soluble polyvalent metal salt are added to a hydrophilic organic liquid. This is because CMC is not dissolved and not dissociated into salt-forming ions. Only when water is added to such a mixture does the CMC dissolve and react with the metal salt to form a gel. Also in this case, however, the gel obtained is not uniform.
We have now determined that it is necessary to make the diffusion velocity of CMC and metal salt into the system higher than the dissolution velocity of CMC into water and the reaction velocity of CMC with metal salt, and found that organic liquids are available as an agent for retarding the dissolution and gelation of CMC.
That is, CMC particles whose surface is covered with a hydrophilic organic liquid are not quickly dissolved or gelated when they are added to an aqueous solution containing a metal salt. In this case, substitution of the organic liquid by water occurs first, and it takes from several seconds to several tens of seconds. Then, the CMC particles covered with water are dispersed and dissolved in water from their surface, and the molecules of CMC are dissolved. The CMC molecules thus dissolved react instantly with the polyvalent metal ions and gelation occurs. Thus there is a period of time of several tens of seconds or more until gelation occurs, after the addition of the CMC is dispersed in a hydrophilic organic liquid into the aqueous polyvalent metal salt solution, and accordingly it becomes possible to effect uniform dispersion and mixing for such period of time, without using any special powerful stirring apparatus, to obtain a uniform and stable gel.